Question: $ D = \left[\begin{array}{rrr}2 & 1 & 0 \\ 2 & 1 & 2 \\ 2 & 0 & 1\end{array}\right]$ What is $ D^{-1}$ ?
$ = \left[\begin{array}{rrr}\frac{1}{4} & -\frac{1}{4} & \frac{1}{2} \\ \frac{1}{2} & \frac{1}{2} & -1 \\ -\frac{1}{2} & \frac{1}{2} & 0\end{array}\right]$